Thermoplastic resins are generally characterized by their many advantageous properties which include optical clarity, high ductility, high heat deflection temperature as well as dimensional stability. As a result of such properties, they are often employed in many commercial applications.
While thermoplastic resins possess the above-described advantageous properties, they often display low abrasion and chemical solvent resistances, and like many other organic polymeric materials, they are susceptible to photodegradation by ultraviolet light. The photodegradation typically results in unfavorable characteristics including yellowing and erosion of the polymer surface.
It is of increasing interest to prepare thermoplastic resins, particularly polycarbonates, that are resistant to abrasion and photodegradation. Such preparation conventionally employs treating the polycarbonate surface with a coating material, whereby the coating material typically contains ultraviolet light absorbing agents such as cyanoacrylate, benzophenone and benzotriazole derivatives.
It is often discovered, however, that the ultraviolet light absorbing agents, themselves, decompose upon exposure to ultraviolet light. This invariably leads to a degradation of the favorable properties of the thermoplastic resin which the agents are originally employed to protect. Additionally, the ultraviolet absorbing agents often have poor compatibility in the coating material which limits the amount which may be employed in the coating.
Moreover, it is further discovered that a number of coating materials do not adhere to the thermoplastic resins with a great deal of ease. To compensate for this, primers are often employed to promote the adhesion of the coating materials to the thermoplastic resins. Thus, it is important for the primers to be clear and colorless and they must adhere to the thermoplastic resins and coating materials without diminishing any of their favorable properties.
The instant invention, therefore, is directed to novel coatings which may also be employed as ultraviolet protective layers as well as primers for silicone hardcoats and the like. The novel coatings comprise photostable polybenzoylresorcinols and they are capable of absorbing ultraviolet light.